1. Field of the Invention
The present invention relates to an apparatus and a process for vacuum forming a material into a predetermined shape. The present invention is most suitable for giving a predetermined shape to a sheet material, such as woven fabric or the like, which has a low elongation ratio.
2. Description of the Related Art
A lid of an automobile console box, for example, includes an armature made of a resin and a sheet coated on a surface of the armature. When manufacturing the lid of the console box, a predetermined shape is given to the sheet in advance by a vacuum forming process, and the sheet thus shaped is then coated on, and fixed integrally to, the resin armature.
In a conventional vacuum forming process, a sheet material is heated to a predetermined thermal deformation temperature range, and pressed onto a molding surface of a mold. At this moment, air remaining between the molding surface of the mold and the sheet material is evacuated through air vent holes, disposed in the molding surface of the mold, by a vacuum source. In this way, the sheet material is brought into close contact with the molding surface of the mold, and shaped in conformance with the molding surface of the mold.
When the sheet material is made of a thermoplastic resin, such as polyvinylchloride (PVC), an acrylonitrile-butadiene-styrene (ABS), or a polyropylene (PP), the sheet material can be easily shaped into a predetermined shape by the conventional vacuum forming process. In addition, a sheet material made of a knitted or non-woven fabric material can be easily formed into a predetermined shape, because such a material has a relatively high elongation ratio (plastic deformation). However, when the sheet material is formed of woven fabric, or the like, with a low elongation ratio, wrinkles often occur on the portions of the sheet material proximate to the corner portions of the molding surface of the mold as illustrated in FIG. 7. Thus, it has been difficult, or impossible, to give a predetermined shape to a sheet material made of woven fabric, or the like, by conventional vacuum forming processes. Hence, when making a sheet from woven fabric or the like, the woven fabric must be cut into a predetermined development shape and sewn together, thereby requiring extensive labor and high cost.